Japanese Patent Publication No. H04-355714A discloses a technique that elongates the interaction length of a light wave and a modulated wave by folding back the optical waveguide of an optical control element at the end face of a substrate, lowers the drive voltage, and compensates the difference of matching speed between the light wave and a signal wave to realize a high-speed operation.
Trials have been made in, for example, “ECOC 2004 PD Th4.2.3” “Compact Zero-Chirp LiNbO3 Modulator for 10-Gb/s Small-Form-Factor Transponder”, which achieve a long interaction length even when a substrate length is shrunk by curving and bending an optical waveguide in a loop form inside an optical modulator.